Porous ceramic bodies and process for their preparation

ABSTRACT

A process for producing a porous ceramic body comprises a) mixing a coated porogen with a silicate or an oxide ceramic precursor, wherein the porogen is decomposable to gaseous decomposition products and optionally solid products upon heating, and is coated with a coating agent; b) forming a green body from the mixture obtained in step (a); and c) firing the green body obtained in step (b) to obtain the ceramic body, whereby the porogen decomposes to form pores within the ceramic body and the coating agent is deposited at the inner surface of the pores. The porogen is coated with a coating agent which, upon firing, is deposited at the inner surface of the ceramic pores, so that porous ceramics having decreased weight and improved porosity are obtained, while maintaining at the same time good mechanical strength. A green body and a porous ceramic body obtainable with the above-mentioned process are described too.

FIELD OF THE INVENTION

The present invention concerns a process for preparing lightweightporous ceramics by using porogens coated with a coating agent; theporous ceramic bodies obtained after decomposition of the porogen uponfiring show an advantageous density reduction without a significantimpairment of the mechanical strength, a more homogeneous total porosityand improved closed porosity.

BACKGROUND OF THE INVENTION

Ceramic materials are commonly provided with a natural porositydetermined by the composition and the grain structure of the rawmaterials, and by the driving out of the water during drying and firing.This natural porosity can be insufficient for application situationswhere it is important to assure a good thermal and electricalinsulation, in addition to refractory properties, and where a reducedweight is required.

Various ceramic products with higher porosity levels have been developedin the art; the incorporation of pores is commonly obtained by means ofso-called porogens, i.e., pore forming materials such as graphite,polymer beads or fibres, which burn, evaporate or gasify upon heatingwithout leaving residues. Porogens are commonly premixed with theceramic precursors and volatilize during the firing step, thus leavingpores in the ceramic body which correspond to the space originallyoccupied by the porogen particles.

The obtained porous ceramics, having reduced density and weight, arecommonly used in a wide range of applications, such as tableware,sanitaryware, tiles, insulators, and various building and constructionmaterials.

For instance, U.S. Pat. No. 5,171,721 describes porous ceramic sheets,which are obtained by mixing a ceramic powder with planar or elongateflakes, such as graphite, starch and sugar, which disappear uponapplication of heat to provide generally planar or elongate porescorresponding to the flakes. The sheets are used as a protective layerfor electrodes. U.S. Pat. No. 6,057,030 discloses porous ceramic bodiescharacterized by a plurality of interconnected pores, obtained by mixinga powdered ceramic component, such as alumina, with pore-formingpolymeric beads, such as polymethylmethacrylate beads, compressing theobtained mixture and finally heating it to vaporize the polymeric beads.

The mechanical strength of the fired ceramic is usually directly relatedto the ceramic's density, i.e. dense ceramics are generally strongerthan porous ceramics. In fact, while the introduction of voids in theceramic material has very positive effects on thermal and electricalinsulation properties of the material and also allows a reduced weight,the presence of voids may also lead to a disadvantageous strengthreduction. Dramatic reduction of modulus of rupture can be experiencedwhen high levels of porosity are introduced in the ceramic, and inparticular open porosity. Open porosity not only reduces melt strength,but also allows water and moisture to penetrate in the ceramic body,which is undesired for many traditional ceramic applications; in fact,open porosity is determined by the pores which are connected to thesurface of the ceramic and linked with each other.

As known in the art, the decrease in mechanical strength may bepartially avoided by controlling homogeneity of the pore distribution.US Statutory Invention Registration H48 of Apr. 1, 1986 describes aporous ceramic article obtained by using thermally degradable beads,such as polystyrene foam beads, in the presence of a binder capable oftaking a set, such as an epoxy resin; after forming the desired ceramicarticle, heat is applied to degrade and volatilize both the beads andthe binder.

US2006/0228247 describes the use of homogenizing agents, such asalcohols, isoparaffinic solvents or polyethyleneglycol, which maintain auniform distribution of the pore-forming agents within a metal or aceramic mass. The pore-forming agents are metal salts which are removedby water extraction after the desired article has been formed, thusavoiding high temperature vaporization or other severe measures that mayalter the character of the final porous article. Nevertheless, theobtained porous ceramics are still unsatisfactory as regards mechanicalstrength.

Further problems associated to the use of high amounts of porogens arecaused by the quick decomposition and combustion of the porogens uponheating, which may lead to cracking in a moulded body. An attempt tosolve this problem is disclosed in US2005/0161849, wherein thepore-forming material is composed of inorganic particles contained inhollow organic polymer particles, such as polymer micro-balloons; theinorganic particles are transferred into the polymer particles bydegassing. The presence of inorganic particles in the pore-formingmaterial decreases the relative amount of organic material, thusavoiding the occurrence of local heat shocks during the heating step.

Therefore, there is a need for developing ceramic porous materialshaving decreased weight and excellent thermal and electrical insulationproperties, while at the same time maintaining good mechanical strength.

SUMMARY OF THE INVENTION

The Applicant has surprisingly found that the above-mentioned problemsmay be solved by using a porogen coated with a coating agent which, uponfiring, is deposited at the inner surface of the ceramic pores; by usingspecific coated porogens, it is possible to obtain porous ceramicshaving decreased weight and improved porosity, while maintaining at thesame time good mechanical strength. In one aspect, the present inventionprovides a process for producing a porous ceramic body, comprising:

a) mixing at least a coated porogen with at least a silicate or an oxideceramic precursor, wherein the porogen is decomposable to gaseousdecomposition products and optionally solid products upon heating, andis coated with a coating agent;

b) forming a green body from the mixture obtained in step (a); and

c) firing the green body obtained in step (b) to obtain the ceramicbody, whereby the porogen decomposes to form pores within the ceramicbody and the coating agent is deposited at the inner surface of thepores.

In another aspect, the invention provides a green body comprising amixture of at least a coated porogen intermixed with at least a silicateor an oxide ceramic precursor, wherein the porogen is decomposable togaseous decomposition products and optionally solid products uponheating, and is coated with a coating agent.

In a further aspect, the invention provides a porous ceramic body havingtotal porosity of at least 10%, open porosity lower than 1.5% and closedporosity of at least 8.5%, wherein the inner surface of the pores iscovered by the thermolysis product of a porogen coated with a coatingagent; the porous ceramic body is obtainable by heating a mixture of atleast a coated porogen with a ceramic precursor, whereby the porogendecomposes to form pores within the ceramic body and the coating agentis deposited at the inner surface of the pores.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention comprises mixing at least a coatedporogen with a ceramic precursor. The porogen used in the process of theinvention must be able to decompose to gaseous and optionally soliddecomposition products upon heating, thus leaving the ceramic poresafter the material is burned out. For example, the porogen can thermallydecompose by burning and/or evaporation.

According to an embodiment, the porogen is a carbon or an organiccompound, preferably selected from the group consisting of graphite,cellulose, starch, organic polymers and mixtures thereof. Suitableorganic polymers comprise homo and copolimers of acrylic and methacrylicacid and esters, styrene, ethylene, propylene and vinylchloride, as wellas polyamides and polyesters.

In some embodiments, the porogen can include carbon or organic sphericalparticles, fibres or mixtures thereof; spherical particles can have adiameter higher than 5 μm, preferably ranging from 10 to 500 μm, andmore preferably from 10 to 100 μm; carbon or organic fibres can have anaverage length ranging from 100 to 1000 μm, and an average width rangingfrom 15 to 40 μm, with a ratio width/length of about 1:5 to 1:50.

The porogen is coated with a coating agent which may be a sintering or areacting component. The coating agent can be a silicate ceramic or anoxide ceramic acting as a sintering component, preferably selected fromthe group consisting of quartz, silica, kaolin, chamotte, mullite,alumina, zirconia, yttrium oxide, titanium oxide, magnesium oxide,calcium oxide, barium oxide and mixtures thereof. Even more preferably,the coating agent is nano-sized alumina, nano-sized silica or mixturesthereof, which highly increase the strength of the final product afterfiring.

When the coating agent is a reacting component, it may be selected fromthe group consisting of feldspar, glass, frits, syenites, aluminiumhydroxide, aluminium oxide hydrate, aluminium nitride, silicon nitride,halides of aluminium, yttrium, zirconium and silicon (preferablychlorides thereof, such as aluminium trichloride, yttrium tetrachloride,zirconium tetrachloride, silicon tetrachloride), zirconium carbonate,salts of alkaline and alkaline earth metals (like calcium carbonate,magnesium carbonate, calcium hydrogen-carbonate, calciumhydrogen-phosphate, calcium-bis-hydrogen-phosphate) and mixturesthereof. Even more preferably, the coating agent is selected from thegroup consisting of aluminium hydroxide, calcium carbonate, magnesiumcarbonate, calcium hydrogen-phosphate and mixtures thereof.

For example, in the case of calcium hydrogen-phosphate, the reactingcomponent may react upon heating to form Ca₂P₂O₇, i.e. artificial boneash. In contrast, CaCO₃ can react upon heating to form CaO and CO₂, thelatter further contributing to the creation of pores.

The coated porogen may be obtained, according to an advantageous method,by adding the coating agent (such as lithium feldspar, calcium carbonateor chalk) to a liquid, such as an alcohol or water, containing theporogen (such as carbon or organic particles and/or fibres) andoptionally a binder system; the obtained suspension is then mixedaccording to conventional methods known in the art.

In case the coating agent is added to an alcohol, the binder system(such as poly-acrylate) is preferably soluble in such alcohol butinsoluble in the medium forming the ceramic precursor slurry (e.g.water). Alternatively, if the coating agent is added to water, it ispreferred to use a binder system, such as long-chained polyvinylalcohol, which is only to soluble at high temperatures (e.g. higher than90° C.) but insoluble at room temperature when forming the ceramicprecursor slurry.

The obtained suspension of coating agent, porogen and optionallybinder-system is dried, so that the coating agent forms a covering layeronto the porogen. The obtained coated porogen is ready to be added to aceramic precursor, preferably in the form of a slurry, e.g. in water;since the binder system is not soluble in water at room temperature, thecoated porogen will not be altered. The obtained slurry can be driedaccording to common procedures, for instance spray dried.

According to an alternative method, the porogen may be added to asolution of calcium hydroxide, obtained by reacting limestone in water;after mixing the obtained suspension, CO₂ is added under controlledheating and stirring conditions, at a temperature preferably not higherthan 80° C., thus precipitating calcium carbonate, which will depositonto the porogen particles. Ceramic components such as kaolin, feldspar,quartz and optionally a binder may be then added to the obtainedmixture; the obtained slurry can be dried according to standardprocedures, for instance spray dried.

According to an embodiment, the amount of coating agent coating theporogen may range from 1 to 40% wt. based on the weight of the coatedporogen, preferably from 3 to 20% wt.

The amount of coated porogen in the mixture obtained from step (a) maybe at least 5% wt., based on the weight of the mixture, preferably from10 to 40% wt., more preferably from 20 to 30% wt.

The silicate or oxide ceramic precursor used in the process of theinvention may be any silicate and oxide ceramic precursors known in theart, and is preferably selected from the group consisting of quartz,silica, feldspar, kaolin, chamotte, mullite, alumina, zirconia, yttriumoxide, titanium oxide, magnesium oxide, calcium oxide, barium oxide andmixtures thereof.

The ceramic precursor may be in the form of powders, whiskers, plateletsor other shapes commonly available in the art.

The ceramic precursor and the coating agent may be the same ordifferent; according to an embodiment, the ceramic precursor isdifferent from the coating agent.

When a dense ceramic product is desired, a sintering aid can be added tothe starting mixture. Useful sintering aids are the ones known in thestate of the art, and may be chosen depending upon the ceramic precursoremployed; for example, CaO, MgO and SiO₂ are suitable sintering aidswhen used with alumina as ceramic precursor.

In step (a) of the process of the invention, the coated porogen and theceramic precursor are mixed according to conventional methods known inthe art; the mixture may optionally contain a dispersant and/or a bindersystem, known in the state of the art.

The coated porogen and the ceramic precursor may be dispersed in waterand mixed, thus obtaining a mixture preferably containing 40 to 60% wt.water. Suitable dispersants may be added to the water mixture, such assodium silicate or sodium hydroxide, to give a pH of about 8-10; thedispersant is preferably contained in an amount ranging from 0.05 to0.5% wt. based on the total weight of the mixture.

Suitable binder systems may be polyvinyl alcohol, polyethylene glycol,polyvinyl acetate, silanes and mixtures thereof, and other binderscommonly used in the art; the binder is preferably contained in anamount ranging from 0.5 to 2% wt. based on the total weight of themixture.

Step (b) of the process of the invention comprises forming a green bodyfrom the mixture obtained in step (a); the green body may be formed bycasting, pressure casting, jiggering, extruding or press processing, allmethods known in the state of the art, and preferably by pressurecasting or press processing.

The green body is preferably formed at a pressure ranging from 200 to3000 bars, at a temperature ranging from 15 to 40° C., more preferablyat 20-25° C. Casting may be carried out at atmospheric pressure, whilepressure casting may be carried out at 20 to 40 bars; press processingmay be carried out at 200 to 350 bars with silicate ceramics, while apressure of 300 to 3000 bars may be applied when press processing oxideceramics.

When casting or pressure casting is used to form the green body, a watermixture obtained in step (a) may be directly employed; differently, whenthe green body is formed by jiggering or extruding, the water mixtureobtained in step (a) is preferably first dewatered to obtain a watercontent of about 18 to 25% wt. before jiggering or extruding. Finally,when the green body is formed by press processing, the water mixtureobtained in step (a) is preferably dewatered to a water content of 3 to10% wt. before press processing.

The green body may be finally dried, to a water content lower than 1%wt. water, based on the total weight of the green body.

Step (c) of the process of the invention comprises firing the green bodyto form a ceramic body, whereby the porogen decomposes to form poreswithin the ceramic body; the coating agent decomposes to solidcomponents and optionally volatile components upon firing, and the solidcomponents are deposited at the inner surface of the pores.

In general, the term “firing” refers to a heat treatment at hightemperature; in the process of the invention, the firing step can becarried out in a conventional and known manner, preferably at atemperature ranging from 150 to 1800° C., for a time ranging from 0.5 to70 hours, more preferably from 1 to 18 hours.

According to an embodiment, the firing step of silicate ceramics iscarried out in two different phases. The first firing phase is carriedout under oxidizing conditions, at a temperature ranging from 200 to1200° C., preferably from 600 to 960° C., for a time comprised between 4and 18 hours, and preferably between 8 and 12 hours; the second firingphase is carried out under oxidizing and reducing conditions, at atemperature higher than 1000° C., preferably from 1200 to 1400° C., fora time comprised between 1 and 12 hours, preferably between 4 and 6hours.

According to another embodiment, the firing step of oxide ceramics iscarried out under oxidizing conditions, at a temperature raging from 150to 1200° C. for a time up to 50 hours, and then at a temperature higherthan 1200° C., preferably from 1600 to 1800° C., for a time comprisedbetween 1 and 18 hours, preferably between 4 and 12 hours.

The firing step can be carried out under oxidizing or reducingatmosphere, by adding oxidizing or reducing agents or other chemicalsubstances in order to exert a combined physical/chemical activation.

When the firing is accomplished under oxidizing conditions, suitableoxidizing atmospheres may be pure oxygen, oxygen/noble gas mixtures andair, among which air is preferably used for economic reasons. Uponfiring under oxidizing atmosphere, the porogen will be decomposed togaseous decomposition products thus creating voids, white the coatingagent coating the porogen will cover at least part of the inner surfaceof the voids. The coating agent has also the function of homogenisingthe coated porogens within the ceramic precursor, thus avoidingmigration and separation of the coated porogen from the ceramic slurry.

When the firing is accomplished under reducing atmosphere, the coatedporogens will decompose to gaseous and solid products; for instance,cellulose particles or fibres will pyrolyse to graphite particles orfibres.

The obtained ceramic body may be glazed according to standardmethodologies known in the art.

The process of the invention allows to obtain lightweight porousceramics of higher quality, having improved porosity, in particularclosed porosity, lowered water adsorption, and improved thermal andelectrical insulation, at the same time maintaining good mechanicalstrength properties, as evident from the bending strength values. Infact, the coated porogens described above can help to obtain a moreuniform and controlled pore size and pore distribution in the ceramicmaterial.

Moreover, without wishing to be bound by theory, it is believed that thecoating agent coating the porogen is deposited at the inner surface ofthe ceramic pores upon firing, thus giving a smooth and regular innerpore surface; the pore surface regularity highly decreases theoccurrence of flaws and cracks. In fact, pore surface irregularities inporous ceramic materials may lead to a non-homogeneous distribution ofthe applied stresses, thus causing a decreased mechanical strength andthe associated occurrence of cracks and failures.

In addition to smoothening the inner pore surface, the coating agentdeposited on the surface of the pore may react in situ, thus closing thepores. In fact, the process of the invention allows to obtain porousceramics having increased closed porosity; the increased closed porosityis not only associated with a higher mechanical strength, but also leadsto lower water absorption levels. Water absorption is undesired andcommonly not accepted for many traditional ceramic applications.

In another aspect, the present invention provides a green bodycomprising a mixture of at least a coated porogen intermixed with atleast a silicate or an oxide ceramic precursor, wherein the porogen isdecomposable to gaseous decomposition products and optionally soliddecomposition products upon heating, and is coated with a coating agent.

The green body preferably comprises more than 5% wt. of coated porogen,preferably from 10 to 40% wt., and more preferably from to 20 to 30% wt.The amount of coating agent coating the porogen may range from 1 to 40%wt., based on the weight of the coated porogen.

In a further aspect, the present invention provides a porous ceramicbody having total porosity of at least 10%, open porosity lower than1.5%, and closed porosity of at least 8.5%, wherein the inner surface ofthe pores is covered by the thermolysis product of a porogen coated witha coating agent; the inner surface of the pores may also be covered bythe reaction product of the coating agent with the ceramic precursor.The porous ceramic body is obtainable by decomposing upon heating aporogen coated with a coating agent, as indicated above.

The porous ceramic body has a total porosity preferably ranging from 15to 30%, and even more preferably from 20 to 25%.

Open porosity is lower than 1.5%, and preferably from 0.01 to 1.0%.

Closed porosity is at least 8.5%, and preferably from 19 to 24%.

Total porosity corresponds to the sum of open porosity and closedporosity. Open porosity refers to pores exposed to the outer surface ofthe ceramic body, and partly linked with each other, normally giving gasand moisture-permeability. Closed porosity comprises closed pores,non-connected with the surface.

The average pore diameter preferably ranges from 10 to 500 μm, and morepreferably from 10 to 100 μm, measured by electron microscope.

The bulk density of the porous ceramic body is preferably lower than 2.4g/cm³, and most preferably ranges from 1.6 to 2.3 g/cm³. The porousceramic bodies obtainable by the process of the invention may have abulk density reduction of at least 8% with respect to the bulk densityof a ceramic body obtained from the same ingredients and under the sameprocess conditions, but without using porogens; the bulk densityreduction is preferably of 12 to 22%.

Due to the porosity nature and ratios, as indicated above, the porousceramic bodies may show levels of water absorption lower than 2% wt.,and preferably ranging from 0.02 to 1.0% wt.

In the green body and in the porous ceramic body of the invention, theporogen may be a carbon or organic compound as indicated above, and ispreferably selected from the group consisting of graphite, cellulose,starch, organic polymers and mixtures thereof.

The coating agent may be as indicated above, and is preferably a ceramicselected from the group consisting of quartz, silica, kaolin, chamotte,mullite, alumina, zirconia, yttrium oxide, titanium oxide, magnesiumoxide, calcium oxide, barium oxide and mixtures thereof. According toanother embodiment, the coating agent may be selected from the groupconsisting of feldspar, glass, frits, syenites, aluminum hydroxide,aluminum oxide hydrate, aluminum nitride, silicon nitride, aluminumhalides, yttrium halides, zirconium halides, silicon halides, zirconiumcarbonate, salts of alkaline and alkaline earth metals, and mixturesthereof. The coating agent is preferably selected from the groupconsisting of aluminium hydroxide, calcium carbonate, magnesiumcarbonate, calcium hydrogen-phosphate and mixtures thereof. The amountof coating agent coating the porogen may range from 1 to 40% wt., basedon the weight of the coated porogen.

The silicate or oxide ceramic precursor of the green body and of theporous ceramic body of the invention is preferably selected from thegroup consisting quartz, silica, feldspar, kaolin, chamotte, mullite,alumina, zirconia, yttrium oxide, titanium oxide, magnesium oxide,calcium oxide, barium oxide and mixtures thereof.

The porous ceramic bodies show improved porosity, lowered wateradsorption, improved thermal and electrical insulation, and goodmechanical strength with respect to the strength of a ceramic bodyobtained from the same ingredients and under the same processconditions, but without using porogens.

The ceramic precursor and the coating agent may be the same ordifferent; according to an embodiment, the ceramic precursor isdifferent from the coating agent.

The bodies of the invention are particularly advantageous for producingtableware, sanitaryware, tiles, bricks, insulaters, building andconstruction materials, as well as technical-silicate ceramics.

The following analytical methods have been used to determine theproperties reported in the detailed description and in the examples.

Property Method Moisture water content (%) wc = [(weight_(wet) −weight_(dry))/weight_(wet)] × 100 Modulus of Rupture 3-point bendingstrength based on DIN 51030, (MOR) on Instron ® instrument (DIN 51223)Bending Strength DIN EN 843-1 (3-point bending strength), on Instron ®instrument (DIN 51223) True Density (ρ_(t)) DIN EN 993/2 (Pyknometer, inwater at 22° C. and 0.3 bar) Open porosity (π_(a)) DIN EN 993/1 + 2 (inwater, at 22° C. and 0.3 bar, with a plastic drier of 25 cm diameter and13 cm height),wherein:

π_(a)=[(m3−m2)/(m3−m2)]×100

the following parameters being determined according to EN 993-1:m1 (g)=mass of dried samplem2 (g)=apparent mass of sample in liquid and infiltrated with thatliquidm3 (g) mass of liquid infiltrated sample in air

Bulk Density (ρ_(b)) DIN EN 993/1 + 2 (by buoyancy, in water at 22° C.and 0.3 bar), wherein: ρ_(b) = [m1/(m3 − m2)] × ρ_(liquid) Totalporosity (π_(t)) DIN EN 993/1 + 2 (in water, at 22° C. and 0.3 bar),wherein: π_(t) = [(ρ_(t) − ρ_(b))/ρ_(t)] × 100 Closed porosity (π_(f))DIN EN 993/1 + 2 (in water, at 22° C. and 0.3 bar), wherein: π_(f) =π_(t) − π_(a) Water absorption DIN EN 1217 (water, at 22° C.) WhitenessDIN 6174

EXAMPLES

The following examples are given for illustrative and not limitingpurposes.

The following commercial products were used in the preparation of thecoated porogens of the process of the invention and in the comparativeexamples:

JRS101: cellulose fibres, having length of 100 μm and medium diameter of20 μm, commercialized by Rettenmeyer.

JRS200: cellulose spherical particles, having diameter of 200 μm,commercialized by Rettenmeyer.

JRS FIC500P: cellulose fibres, having length of 500 μm and mediumdiameter of 30 μm, commercialized by Rettenmeyer.

CaCO3: Calcit, Type K6/L II commercialized by Franz Mandt GmbH, particlesize D50 of 5.6 μm.

The coated porogens used in the process of the invention were obtainedas follows.

Porogen 1

In a 3-neck round glass-bottle, 1.8 g of polyvinyl alcohol(Polyviol^(R), commercialized by Wacker Polymer Systems GmbH &Co. KG)was added to 100 ml water, and the mixture was heated at about 95° C.under stirring for 4 hours, using an oil bath with controlled heatingdevice, thus dissolving the alcohol. To the obtained solution, 23 gJRS101 were added under stirring, followed by 2 g calcium carbonate,maintaining the mixture under stirring. The obtained mixture was driedby heating up to 100° C. and distilling the water by using a distillingdevice, thus causing calcium carbonate and polyvinyl alcohol to depositonto the porogen.

Porogen 2

The coated porogen was prepared according to the procedure reportedabove for Porogen 1, with the exception that 5 g calcium carbonate wereadded.

Porogen 3

In a 3-neck round glass-bottle, 1.8 g of polyacrylate (Sokalan^(R) PA40, commercialized by BASF) was added to 100 ml iso-propyl alcohol anddissolved under stirring for 2 hours at 30° C. To the obtained solution,23 g JRS101, based on the weight of the solution, was added understirring, followed by 20% wt. calcium carbonate (based on the weight ofthe fibres), maintaining the mixture under stirring. The obtainedmixture was dried by heating up to 85° C. and distilling the by using adistilling device, thus causing calcium carbonate and polyacrylate todeposit onto the porogen.

Porogen 4

The coated porogen was prepared according to the procedure reportedabove for Porogen 1, with the exception that JRS 200 and 5 g calciumcarbonate were used.

Porogen 5

The coated porogen was prepared according to the procedure reportedabove for Porogen 3, with the exception that JRS200 was used.

Porogen 6

In a 3-neck round glass-bottle, 5.6 g limestone were added via a funnelto 500 ml water, maintained under stirring at 40° C. by means of an oilbath with controlled heating device; calcium oxide reacted to calciumhydroxide. To the obtained solution, 46.0 g of MS FIC500P were added viaa funnel, and CO₂ was added under mixing by means of a gas bottle withreducing valve (2-3 bubbles per sec), thus obtaining a suspension. Thetemperature was increased of 15° C./hour up to a temperature of 80° C.,and the mixture was maintained at such temperature under stirring for 3hours, thus precipitating 9.2 g calcium carbonate, which deposited onthe fibres. The obtained mixture was cooled to room temperature and thecoated porogen was isolated.

Examples 1-6

The standard mixture for porcelain PP910B, commercialized by ImerysTableware Deutschland GmbH (comprising 57% wt. of a mixture of kaolins,42% wt. pegmatite and 1% wt. polyvinyl alcohol) was used as ceramicprecursor; the porcelain mixture of Example 6 was the same, butcontaining about 2% wt. polyvinyl alcohol (Polyviol^(R), commercializedby Wacker Polymer Systems GmbH & Co. KG).

50 g of said mixture (154 g in Example 6) were added under stirring to70 ml of water (300 ml in Example 6) thus forming a slurry; the porogensreported in Table 1, in the amounts indicated therein, were added to theslurry, and the resulting mixture was homogenized by mixing.

100 g of the ceramic slurry were dried cautiously at 60° C., maintainingthe mixture homogeneous under stirring. The dried ceramic cake, having amoisture content of about 7-8% wt., was crushed to small particles(average diameter lower than 300 μm) and sieved on a screen of 500 μm.The thus obtained ready-to-press powder was pressed in a mould intogreen bars (30 bars, having size of 100×20×7 mm), by applying a pressureof 300 bars.

The moisture and Modulus of Rupture (MOR) of the obtained green barswere determined for each example, as reported in Table 1.

The green bars were then dried at 110° C. and fired at 950° C. underoxidising atmosphere, for 12 hours. The bars were fired at increasingtemperature up to 1100° C., in 2 hours, under oxidizing atmosphere, andthen under reducing atmosphere at temperatures higher than 1100° C. upto 1400° C., for 4 hours.

The bulk and true density, total porosity, open porosity, closedporosity, bending strength, water adsorption and whiteness of theobtained porous ceramic bodies were characterised for each example, asreported in Table 2.

Comparative Examples 1-5

The procedure of Example 1 was repeated by using, instead of the coatedporogens of the process of the invention, the products indicated inTable 1, in the amounts reported therein.

The data reported in Table 2 show that the porous ceramic bodies of theinvention achieve a relevant reduction in density, at the same timemaintaining good levels of bending strength and the desired levels ofwater adsorption.

More specifically, the ceramic bodies of the invention show a desirabledensity reduction as compared to ceramic bodies obtained in the absenceof coated porogen (Comp. Ex. 1) or with CaCO₃ (Comp. Ex. 2); moreover,the process of the invention leads to ceramic bodies maintaining goodlevels of bending strength, while the use of uncoated porogens has verynegative effects on mechanical strength and water absorption levels(Comp. Ex. 3-5). Although only preferred embodiments of the inventionare specifically disclosed and exemplified above, it will be appreciatedthat many modifications and variations of the present invention arepossible in the light of the teachings, within the scope of theinvention as defined by the appended claims.

TABLE 1 Coating agent Porogen amount amount Moisture MOR Example Porogen(% wt.) (% wt.) (% wt.) (MPa) Example 1 Porogen 1 2.0 23 7.2 1.01Example 2 Porogen 2 4.8 23 6.9 1.02 Example 3 Porogen 3 20.0 23 7.4 0.91Example 4 Porogen 4 4.8 23 7.5 0.91 Example 5 Porogen 5 20.0 23 10.80.78 Example 6 Porogen 6 20.0 23 6.2 0.46 Comp. Ex. 1 — 0 0 6.3 0.90Comp. Ex. 2 CaCO₃ 0 4.8 7.7 0.85 Comp. Ex. 3 JRS101 0 23 7.7 0.90 Comp.Ex. 4 JRS 200 0 23 9.6 0.74 Comp. Ex. 5 JRS FIC500P 20.0 23 7.6 0.87

TABLE 2 True Bulk Total Open Closed Bending Water Density Density BulkDensity Porosity Porosity Porosity Strength adsorption Example (g/cm³)(g/cm³) reduction (%) (%) (%) (%) (MPa) (% wt.) Whiteness Example 1 2.482.17 12 12.5 1.0 11.5 50 0.44 72.4 Example 2 2.48 2.17 12 12.5 0.2 12.348 0.08 72.6 Example 3 2.49 2.23 10 10.4 0.2 10.3 49 0.08 73.5 Example 42.47 1.93 22 21.9 1.0 20.8 34 0.55 72.5 Example 5 2.49 2.23 10 10.4 0.110.3 50 0.06 73.5 Example 6 2.49 2.29  8 10.0 0.3 9.7 51 0.14 72.2 Comp.Ex. 1 2.50 2.47 — 2.4 0.1 2.0 62 0.04 69.0 Comp. Ex. 2 2.47 2.30  7 6.90.1 6.8 54 0.04 72.9 Comp. Ex. 3 2.47 1.92 22 22.3 19 3.2 31 9.90 72.0Comp. Ex. 4 2.47 1.77 28 28.3 22.5 5.8 24 12.7 70.4 Comp. Ex. 5 2.481.61 24 35.6 17.1 18.5 34 11.5 72.4

1. A process for preparing a porous ceramic body, comprising: a) mixingat least a coated porogen with at least a silicate or an oxide ceramicprecursor, wherein the porogen is decomposable to gaseous decompositionproducts and optionally solid products upon heating, and is coated witha coating agent; b) forming a green body from the mixture obtained instep (a); and c) firing the green body obtained in step (b) to obtainthe ceramic body, whereby the porogen decomposes to form pores withinthe ceramic body and the coating agent is deposited at the inner surfaceof the pores.
 2. The process of claim 1, wherein said porogen is acarbon or organic compound.
 3. The process of claim 1, wherein saidporogen is a spherical particle or a fibre.
 4. The process of claim 2,wherein said porogen is selected from the group consisting of graphite,cellulose, starch, organic polymers and mixtures thereof.
 5. The processof claim 1, wherein said coating agent is a ceramic selected from thegroup consisting of quartz, silica, kaolin, chamotte, mullite, alumina,zirconia, yttrium oxide, titanium oxide, magnesium oxide, calcium oxide,barium oxide and mixtures thereof.
 6. The process of claim 1, whereinsaid coating agent is selected from the group consisting of feldspar,glass, frits, syenites, aluminum hydroxide, aluminum oxide hydrate,aluminum nitride, silicon nitride, aluminum halides, yttrium halides,zirconium halides, silicon halides, zirconium carbonate, salts ofalkaline and alkaline earth metals, and mixtures thereof.
 7. The processof claim 6, wherein said coating agent is selected from the groupconsisting of aluminium hydroxide, calcium carbonate, magnesiumcarbonate, calcium hydrogen-phosphate and mixtures thereof.
 8. Theprocess of claim 1, wherein the coating agent is different from theceramic precursor.
 9. The process of claim 1, wherein the amount ofcoating agent coating the porogen ranges from 1 to 40% wt., based on theweight of the coated porogen.
 10. The process of claim 1, wherein theamount of coated porogen in the mixture of step (a) is at least 5% wt.,based on the weight of the mixture.
 11. The process of claim 10, whereinthe amount of coated porogen ranges from 10 to 40% wt.
 12. (canceled)13. The process of claim 1, wherein in step (b) the green body is formedby casting, pressure casting, jiggering, extruding or press processing.14. The process of claim 1, wherein in step (c) the firing is carriedout at a temperature ranging from 150 to 1800° C., for a time rangingfrom 0.5 to 70 hours.
 15. The process of claim 1, wherein the ceramicbody obtained in step (c) has total porosity of at least 10%, openporosity lower than 1.5% and closed porosity of at least 8.5%.
 16. Agreen body comprising a mixture of at least a coated porogen intermixedwith at least a silicate or an oxide ceramic precursor, wherein theporogen is decomposable to gaseous and optionally solid decompositionproducts upon heating, and is coated with a coating agent.
 17. The greenbody of claim 16, comprising from 10 to 40% wt. of coated porogen. 18.The green body of claim 16, wherein the amount of coating agent coatingthe porogen ranges from 1 to 40% wt., based on the weight of the coatedporogen.
 19. A porous ceramic body having total porosity of at least10%, open porosity lower than 1.5% and closed porosity of at least 8.5%,wherein the inner surface of the pores is covered by the thermolysisproduct of a porogen coated with a coating agent.
 20. The porous ceramicbody of claim 19, having total porosity ranging from 15 to 30%.
 21. Theporous ceramic body of claim 19, having open porosity ranging from 0.01to 1.0%. 22-24. (canceled)
 25. The porous ceramic body of claim 19,having a bulk density reduction of at least 8%.
 26. The porous ceramicbody of claim 19, having water absorption lower than 2% wt.